1. Field of Invention
The invention relates to a power source protection circuit, and more particularly to a three-in-one protection circuit against an out-rush circuit, short circuit and overload circuit.
2. Technical Background
Normally, a power supply has its own rated output power, output voltage and output current. The power supply can work normally in these rated working ranges. A protection circuit for protecting the circuit of the power supply must be designed to prevent operation over the rated work range. For example, the power supply needs to provide an out-rush current when a large-scale electromagnetic contactor is activated. This current is several times (1.5 to 3 times or higher) larger than the overload current and many more times (approximately 7 times or higher) larger than the rated current. Therefore, the protection for these out-rush and short circuits is always considered when a power supply is designed.
Please refer to FIG. 1, which shows the circuit block diagram of the conventional protection circuit against an out-rush current and overload. The voltage variation characteristics of the subordinate coil of a transformer 103 are used to design the circuit. The protection circuit comprises a few parts: an executor for activating protection function 107, a judgment device 108, a first voltage level generator 111, a second voltage level generator 104, a first timer 110, a second timer 105 and an executor for keeping system shut down 109. Other parts such as the AC power source 101, the rectifier 102, the transformer 103 and control system 106 are the main elements for a normal power supply.
Here, the executor for activating protection function 107 is used to force the control system to shut down. The judgment device 108 is responsible for judging whether protection against an out-rush current and overload current is processed. The T2 timer 110 is used to limit the output time of an out-rush current and overload current. The executor107 is not informed and the system does not enter into the protection state if the counting of the timer is not completed. The first voltage level generator 111 generates an accurate voltage level to the executor 107 so that its internal comparator therein has a reference level for comparison. The executor 109 informs the executor 107 of shutting down the control system 106. The second timer 105 is used to limit the shutdown time of the control system 106. The second voltage level generator 104 generates an accurate voltage level to the executor 109 so that its internal comparator therein has a reference level for comparison.
The operation of the whole circuit is described as follows:
The output voltage of the subordinate circuit of the transformer becomes larger when the output current increases continuously. The judgment device 108 determines whether the protection is activated according to the output current or output voltage of the transformer 103. The executor 107 would activate the first timer 110 if the protection circuit needs to be activated. When the first timer 110 completes counting and the output current is still larger than the predetermined overload current level, the executor 107 would then shut down the control system 106. The executor 109 is activated simultaneously to force the executor 107 to keep shutting down the control system 106. These two executors form a cooperation relationship (mutually locking). The second timer 105 starts counting after the executor 109 is activated. The control system 106 would stay in a shutdown state during the counting period. The protection function is disabled and the control system 106 would try to activate the system again after the counting of the second timer 105 is completed. The output voltage and current appear as square waves if the output current is still larger than the predetermined overload current level.
The protection circuit mentioned above is mainly designed for the protection of an out-rush circuit and overload circuit, but does not consider the short circuit. As a result, when short-circuit happens, the power supply would keep generating huge output current and result in severe heat dissipation, and damage some parts accordingly. It would be better to integrate a short-circuit protection function into the protection circuit of the power supply.
The invention provides a three-in-one protection circuit against an out-rush circuit, short circuit and overload. It can protect a power supply from being damaged due to a short circuit, the occurrence of an out-rush current or an overload.
To achieve the above-mentioned purpose, a three-in-one protection circuit of the invention is applied in a power supply with a transformer and a control system. It comprises a second power source, a current sensor, a voltage level generator, a timer and an executor for activating protection function. The second power source, the current sensor and the voltage level generator are all connected to the output side of the transformer. The timer receives the signal corresponding to output current from the current sensor. The executor for activating the protection function is connected to the control system, the second power source, the voltage level generator and the timer.
The timer starts counting when the power supply outputs an out-rush current. The larger the output current comes, the shorter the timer counts. A shutdown signal is sent to the control system when the predetermined time is up. When a short circuit occurs at the output end of the power supply, the voltage level from the voltage level generator decrease to zero and makes the executor for activating the protection function send a shutdown signal to the control system. When the output current is larger than the predetermined overload current level, it would start counting and would generate an overload protection control signal to the executor for activating protection function when the predetermined time is up. The executor for activating protection function also sends a shutdown signal to the control system when it receives the above overload protection control signal.
The second power source provides electricity after the executor sends the shutdown signal; the control system would try to start working when the energy of the second power source is consumed and the protection function is then disabled.
Moreover, the invention provides another three-in-one protection circuit against an out-rush current, short circuit and overload. It comprises a second power source, a current sensor, a first voltage level generator, a signal amplifier, a noise filter, a first timer, an executor for activating protection function, a second voltage level generator, an executor for keeping the system shut down and a second timer. The second power source, the current sensor and the first voltage level generator are all connected to the output side of the transformer. The first timer is used to receive the output current signal that is transmitted from the current sensor to the signal amplifier and the noise filter. The executor for activating the protection function is connected to the control system, the second power source, the first voltage level generator and the first timer.
The first timer starts counting when the power supply generates an out-rush current. A shutdown signal is sent to the control system when the predetermined time is up. The voltage level from the first voltage level generator decreases to zero when a short circuit occurs at the output end of the power supply, and the executor for activating the protecting function sends a shutdown signal to the control system. Counting is started when the output current is larger than the overload current, and an overload protection control signal is generated when the predetermined time is up. The executor for activating the protection function also sends a shutdown signal to the control system when it receives the above overload protection control signal.
The executor for keeping the system shut down is connected to the control system, the executor for activating the protection function and the second voltage level generator. It shuts down the system when it receives the shutdown signal transmitted from the executor for activating the protection function. The second timer connected to the executor for activating the protection function and the executor for keeping the system shut down starts counting when the executor for activating the protection function sends out a shutdown signal. The executor for keeping the system shutting down sends a work-starting signal to the control system when the time is up, and the control system would try to start working.
Also, the second power source comprises a rectifier circuit and a RC discharger circuit. The timer can be the RC charger or discharger circuit. The current sensor can be a resistor.